The present invention relates to a magnetic recording medium, and in particular relates to a magnetic recording medium used as a computer external storage device or the like.
Lubricants have been developed for magnetic recording media, and in particular for magnetic disks, with the purpose of reducing the friction force occurring between the protective layer and the head, and improving durability and reliability.
For example, in order to improve the lubricating characteristics on the surface layer of a magnetic recording medium, perfluoro polyether lubricants having hydroxyl groups or other polar terminal groups or cyclotriphosphazene terminal groups in the molecule are applied onto a diamond-like carbon (DLC) protective layer.
On the surface of a diamond-like carbon (DLC) protective layer, there exist numerous carboxyl groups, hydroxyl groups, amine groups, and other functional groups; terminal groups of the lubricant are actively adsorbed onto or bond with these functional groups.
However, these functional groups have characteristics such that there is active adsorption of water, acidic gases, and other contaminants, in addition to the terminal groups of a lubricant. Hence when there exist residual groups not bonded to lubricant terminal groups on the surface of the protective layer, there is the concern that the amount of adsorbed contaminants may be increased.
Hence in order to suppress adsorption of water, acidic gases, and other contaminants onto the disk medium surface, it is necessary to raise the bonding amount (bonding fraction) of functional groups existing on the carbon surface with the lubricant (see Patent Reference 1 and Patent Reference 2, identified below).
In addition, in recent years there have been urgent demands for higher recording densities, and a need has arisen for a magnetic recording medium having areal densities exceeding 1 Tbits/in2. To achieve this, the particle sizes in the magnetic layer must be reduced and noise must be decreased. However, if particle sizes are made too small, there is the problem that magnetic signals recorded in the magnetic layer are annihilated through thermal demagnetization. Hence in order to raise recording densities, magnetic layers having high thermal stability and a high coercivity must be used.
Hence methods have been proposed in which the region for recording of a magnetic recording medium having a high-coercivity recording layer is heated, for example, by irradiation with laser light to lower the coercivity, and a magnetic head is used to apply a magnetic field according to the information in the region with lowered coercivity, to record the information (see Patent Reference 3 and Patent Reference 4). The above recording method is called thermally assisted magnetic recording.
In the thermally assisted magnetic recording method, by irradiating the recording surface with laser light, the recording layer is heated to approximately 150° C. to 200° C. Hence the above-described lubricant is exposed to high temperatures due to thermal conduction from the recording layer.
However, lubricants of the prior art are not considered for use in the high-temperature environments anticipated in thermally assisted magnetic recording, and there are concerns that in the high-temperature environment described above, the lubricant may be reduced due to volatility.
Patent Reference 1: Japanese Patent Application Laid-open No. H5-247200
Patent Reference 2: Japanese Patent Application Laid-open No. 2004-253110
Patent Reference 3: Japanese Patent Application Laid-open No. 2003-45004
Patent Reference 4: Japanese Patent Laid-open No. 2006-12249
Patent Reference 5: Japanese Patent No. 3223238
Patent Reference 6: U.S. Pat. No. 5,959,058.